1. Field of the Invention
The present invention generally relates to an electrically powered brake system for use in an automotive vehicle.
2. Description of the Prior Art
The power plant employed in automotive vehicles appears to go through changes from the internal combustion engine that consumes petroleum as a fuel to the hybrid power plant comprised of an internal combustion engine and an electric drive motor that are selectively brought into operation one at a time depending on the operating conditions, or to the power plant comprised of only an electric drive motor.
For the brake device employed in automotive vehicles, an alternative to the hydraulically operated brake device has been suggested, which comprises an electrically powered brake device including an electric drive motor utilized to drive a brake piece. In allowing the brake piece to be linearly moved reciprocatingly between activated and released positions, some of the electrically powered brake devices hitherto suggested make use of a ball screw mechanism (known as a ball-and-nut mechanism) for translating a rotary motion into a rectilinear reciprocating motion. See, for example, the Japanese Laid-open Patent Publication No. 2002-257165.
In the conventional hydraulically operated brake device, an anti-skid brake system is in most instances employed so that during the braking the hydraulic pressure can be regulated in reference to the number of revolutions of at least one vehicle wheel to avoid a premature locking of the vehicle wheel, thereby achieving a proper brake control.
Considering that the conventional anti-skid brake system is generally applied to the hydraulically operated brake device so that the available braking force can be adjusted by regulating the hydraulic pressure, the response thereof is relatively low. Because of this, it has been desired to increase the response that is exhibited when the braking force is adjusted relative to detection of the number of revolutions of the vehicle wheel. Also, the hydraulically operated brake device requires the use of hydraulic equipments such as, for example, a hydraulic pump, a drive motor for driving the hydraulic equipments and associated hydraulic supply and drain pipes and, accordingly, simplification and compactization of an automotive vehicle tend to be hampered.
While to simplify the hydraulic system, double or single hydraulic systems are generally employed in one automotive vehicle, failure of the hydraulic pump and/or the drive motor to operate properly may adversely affect the entire number of vehicle wheels that are placed under the control of the same hydraulic system. This is equally true of the anti-skid control which applies the same control to the entire number of vehicle wheels that are placed under the control of the same hydraulic system. For this reason, no control can be performed that is appropriate to the behavior, i.e., the number of revolutions of each of the four vehicle wheels that tends to vary from one wheel to another during, for example, cornering of the automotive vehicle.
On the other hand, various suggestions hitherto made of the electrically powered brake devices are all directed to component parts of the brake device and no suggestion have yet been made of control features thereof.